Electrical slip rings are used to transfer electrical power and/or signals between a rotor and a stator. These devices are used in many different military and commercial applications, such as solar array drive mechanisms, aircraft and missile guidance platforms, undersea robots, CATSCAN systems, and the like. In some of these applications, slip rings are used in conjunction with other rotary components, such as torque motors, resolvers and encoders. Electrical slip rings must be designed to be located either on the platform axis of rotation, or be designed with an open bore which locates the electrical contacts off-axis. Hence, the designations “on-axis” and “off-axis” slip rings, respectively. The diameter of slip ring rotors may range from a fraction of an inch to multiple feet, and the angular speed may vary from one revolution per day to as much as 20,000 revolutions per minute. In all of these applications, the electrical contacts between the rotor and stator must: (1) transfer power and/or signal(s) without interruption at high surface speeds, (2) have long wear life, (3) maintain low electrical noise, and (4) be of a physical size that allows multiple circuits to be packaged in a minimum volume.
The most efficient management of the electrical and mechanical contact physics allows the most demanding requirements to be met. For example, if the application is an off-axis slip ring that allows the x-ray tube in a CATSCAN gantry to rotate about the patient's body, the electrical contacts must be designed to carry about 100-200 amps (with surges of hundreds of amps), operate at surface speeds on the order of 500 inches per second, last for 100 million revolutions, and occupy a minimal volume within the gantry. In order to meet the 100 million revolution requirement for a device that is about six feet in diameter, the brush force must be low to minimize frictional heating and to maintain a large number of contact points between brush and ring to achieve the required current density.
Four types of electrical contacts between a rotor and stator include: (1) a composite solid material brush on a cantilevered spring, (2) a monofilament metal alloy brush that tangentially engages the rotor, (3) a fiber brush having a plurality of individual fibers, with the bundle tangentially engaging the rotor, and (4) a tip-of-fiber contact between the brush and rotor. The contact force, surface speeds and type of lubrication for each contact type is summarized in Table I. Table I also shows the types of lubricants heretofore typically required to reduce the contact frictional heating if the brush force is above one gram.
TABLE ISurfaceType ofContact TypeContact ForceSpeedsLubricationcomposite brush0.4kg/cm2700 in/secsacrificial graphitefilm*monofilament3–20grams 12 in/secboundary lubrication**metal alloytangential fiber1–3grams200 in/secadventitious***brushfiber-on-tip0.1–1grams1200 in/sec adventitious****With a sacrificial graphite film, the brush and ring interface is lubricated by a film of graphite that is transferred from the brush to the ring. Material that is worn away is replaced by graphite from the brush.**With boundary lubrication, a boundary lubricant film supports a portion of the load between the contact members. The points of metal contact support the remaining load between the contact members, and provide the current-carrying capability.***With adventitious films, very thin films of materials that are capable of reducing the coefficient of friction between the contact members under light loads.
The tribological properties of electrical contacts and the right choice of lubricant to meet the requirements of the application are extremely important. For example, if the contacts are to be used in a space application, then the lubricant must meet all of the requirements of a ground-based application, and have a low vapor pressure. If the contacts have a long life requirement, then dust, wear debris and other contaminants may accumulate in the contact zone and create problems with life and signal transfer. However, if the electrical contact members can be brought together with a force of about one gram or less, then the lubricant and the associated complications are eliminated.
For several years, fiber brushes with a tangential orientation to the ring have been successfully used to meet high surface speeds without the use of a lubricant.
Under certain conditions, it may be desirable to provide lubrication during the initial brush run-in process. Moreover, the added fluid may be a lubricant, or a chemical vapor emitter, or an anti-corrosion inhibitor, or a material having a high dielectric strength to retard electrical arcing.
Accordingly, the principal object of the invention is to provide a fiber-on-tips brush with a fluid-dispensing reservoir for selectively dispensing a fluid, particularly during the brush run-in process.